Devices such as cellular phones, digital cameras, PDA, or touch panel displays show a tendency of further prevalence.
Glass substrates used in those applications are required to have high mechanical strength. Conventionally, glass substrates tempered by ion exchange and the like (so-called tempered glass substrate) are used in those applications (see Patent Document 1, Non-patent Document 1).    Patent Document 1: JP 2006-83045 A    Non-patent Document 1: Tetsuro Izumitani et al., “New glass and physicality thereof”, First edition, Management System Laboratory. Co., Ltd., 20 Aug. 1984, p 451-498